An MRI apparatus is an apparatus which measures a nuclear magnetic resonance (hereinafter, referred to as NMR) signal generated by the subject, especially, the nuclear spins which form human tissue and images the shapes or functions of the head, abdomen, limbs, and the like in a two-dimensional manner or in a three-dimensional manner. In the imaging, different phase encoding and different frequency encoding are given to NMR signals by the gradient magnetic field, and the NMR signals are measured as time series data. The measured NMR signals are reconstructed as an image by a two-dimensional or three-dimensional Fourier transform.
In order to perform imaging with the MRI apparatus, it is necessary to use a gradient magnetic field changing with time as described above. The application time and the strength of the gradient magnetic field need to be accurately controlled in order to correctly select an imaging region or correctly give the positional information to an NMR signal. However, a damping current (so-called eddy current) is induced in various structures around the gradient coil due to application of the gradient magnetic field, and this eddy current generates a magnetic field which changes spatially and temporally. When the magnetic field caused by the eddy current reaches an imaging region of a subject together with the gradient magnetic field, the application time and the strength of the gradient magnetic field applied to the nuclear spins in the imaging region deviate from the desired application time and strength. As a result, it becomes impossible to correctly select an imaging region or correctly give the positional information to an NMR signal, and this causes image quality degradation, such as image distortion, a reduction in signal strength, and the occurrence of ghosting.
Therefore, PTL 1 discloses a method of suppressing the image quality degradation due to an eddy current by measuring a magnetic field induced by the eddy current and applying a compensation magnetic field for negating the magnetic field using a shim coil. Specifically, test gradient magnetic fields are applied using two test gradient magnetic fields with different polarities, and then a high-frequency magnetic field pulse and a phase encoding gradient magnetic field are applied to a phantom to measure a free induction damping signal (FID signal). Phase information included in two FID signals acquired is influenced by the magnetic field caused by the eddy current, the phase encoding gradient magnetic field, and the unevenness of the static magnetic field. Therefore, by taking a difference in phase information obtained by a Fourier transform of the two FID signals measured by applying the test gradient magnetic fields with different polarities, the influence by the phase encoding gradient magnetic field and the unevenness of the static magnetic field is eliminated. As a result, a phase difference image representing the influence of the magnetic field due to the eddy current generated due to the test gradient magnetic field is obtained. On the basis of this phase difference image, the spatial distribution and the temporal change of the magnetic field caused by the eddy current are calculated, and the distribution and size of a compensation magnetic field are determined.